1. Field of the Invention
The present invention relates to an apparatus for sychronously demodulating position information from a gyroscope, but more specifically it relates to an apparatus for transforming a cage coil signal portion of the position information which is in a polar coordinate format into a more useful cartesian coordinate format.
2. Description of the Prior Art
The present invention is contemplated for use with an associated gyroscope capable of generating from a cage coil portion thereof a cage coil signal consisting of an amplitude and phase modulated waveform having a carrier frequency which is the same as the inertial spin rate of the rotor of the associated gyroscope. Normally, the cage coil is disposed on the rotor of the gyroscope so that the amplitude of the cage coil signal is proportional to an angle .lambda. and the phase thereof is equal to an angle .theta.. These two angles define the position of the rotor of the gyroscope in a polar coordinate system where the angle .lambda. is a measure of the rotor deflection relative to an axis which is coaxial with the cage coil and passes through its radius center. The angle .theta. is the solid angle determined by the two planes normal to the cage coil. One plane passes through the center of the rotor normal thereto, and the other plane, i.e., a spin reference plane, passes through a spin reference sensor disposed on the rotor. The spin reference plane also passes through the centroid of the cage coil. This plane is also termed the .theta.-reference plane. The foregoing is a description of a typical gyroscope having a cage coil which generates position information in terms of a cage coil signal.
In many guidance applications, it is desirable to measure the gyroscope position with respect to a cartesian coordinate system established in the .theta.-reference plane. The gyroscope position can then be described by .lambda. cos .theta. and .lambda. sin .theta.. The cartesian x and y components then arise from the projection of .lambda. into the cartesian coordinate plane and its subsequent resolution into the x and y components.
In the methods employed by the prior art to measure .lambda., the cage coil signal is sampled after a fixed interval following the zero crossings thereof. The time interval used is T/4, where T is the period of the cage coil waveform. This technique is subject to error since the period of the cage coil waveform varies in most gyroscope applications. This is due to the fact that the frequency of the cage coil waveform changes if the vehicle containing the gyroscope undergoes rotation in the plane of the gyroscope's rotor. Errors will also arise when the speed of the rotor varies as is the case in spin down gyroscope applications. Hence, there is a need in the prior art to eliminate the errors in calculation due to frequency perturbations in the cage coil waveforms and to significantly desensitize the demodulation process to errors caused by varying rotor speed rates.
Yet another source of error in the measurement of .lambda. arises due to the necessity of having to instantaneously sample the cage coil waveform. Accordingly, any noise present on the cage coil waveform at the instant of sampling will become part of the measured response. This noise may be uncorrelated electrical interference. But more likely than not, the noise will be correlated signals which arise due to portions of the spin waveforms and precession waveforms of the associated gyroscope being coupled to the cage coil. The foregoing interfering waveforms have the same fundamental frequency as the cage coil waveform and they usually possess harmonics as well. The result is that the interfering signals slowly "walk" through the peaks of the cage coil waveform as it is sampled. In addition, these noise sources may also introduce error by causing the zero crossings of the cage coil waveform to occur at incorrect times. Consequently, there is a need in the prior art to reduce the effect of interference signals, i.e., noise, in the accuracy of measuring the value of .lambda..
The conventional technique for the measurement of the cos .theta. and the sin .theta. is similar to the measurement of .lambda.. The spin reference waveforms from the spin reference sensors are sampled after a delay of T/4 following zero crossings of the cage coil signal. The sampled values of the spin reference waveforms are equal to the cos .theta. and the sin .theta. for the x and y axes spin reference sensors, respectively. Errors in the zero crossing time of the cage coil signal caused by noise will also affect the measurement of cos .theta. and sin .theta.. Thus, there is a need in the prior art to reduce the effect of noise in the measurement of cos .theta. and sin .theta. in an improved manner.
As outlined hereinabove, the known prior art demodulation methods are quite susceptible to cage coil waveform noise and variations in the spin speed of the rotor of the associated gyroscope. The unwanted coupling of a portion of the spin and precession waveforms into the cage coil waveform is particularly troublesome. Additionally, prior art measurement accuracies are highly dependent on the actual value of .lambda.. Accordingly, there is a need, as related to the foregoing types of systems, to reduce substantially the required accuracy of the demodulation process on the actual value of .lambda..
The prior art, as indicated hereinabove, include various apparatuses for demodulating position information from cage coil signals. However, insofar as can be determined, no prior art apparatus incorporates all of the features and advantages of the present invention.